In the study on organic electroluminescent (EL) devices (hereinafter, simply referred to as an ‘organic EL device’) from the start point of observation of an organic thin film luminescence by Bernanose in the 1950s subsequent to blue electroluminescence using anthracene single crystals in 1965, an organic EL device having a layered structure which is divided into functional layers such as a hole layer and a light emitting layer was suggested by Tang in 1987, and development has been made toward introducing each characteristic organic layer into a device in order to manufacture an organic EL device with high efficiency and long service life, thereby leading to the development of specialized materials used.
The organic EL device is composed of an indium tin oxide (ITO) substrate, an anode, a hole injection layer selectively receiving holes from the anode, a hole transporting layer selectively transporting holes, a light emitting layer emitting light through recombination of holes and electrons, an electron transporting layer selectively transporting electrons, an electron injection layer selectively receiving electrons from a cathode and the cathode.
In a typical organic EL device, when electric energy is applied to the device, holes from an anode and electrons from a cathode are injected into organic material layers through each functional layer between the anode and the cathode. The device is operated base on the principle that when the injected holes combine with the injected electrons, excitons are formed, and then, when the exciton falls to the ground state again, light is emitted.
The organic EL device is manufactured into a multilayered structure because the transporting speeds of holes and electrons are different from each other, and if a hole injection layer, a hole transporting layer, an electron transporting layer and an electron injection layer are properly prepared, holes and electrons may be effectively transported and the balance between holes and electrons in the device may be achieved, thereby enhancing luminous efficiency.
Electrons injected from the electron injection layer and holes transported from the hole injection layer recombine in the light emitting layer to form an exciton, and the luminescence resulting from the exciton falling from the singlet excited state to the ground state refers to fluorescence, and the luminescence resulting from exciton falling from the triplet excited state to the ground state refers to phosphorescence. When carriers recombine in the light emitting layer to generate excitons, singlet and triplet excitons are generated theoretically in a ratio of 1:3, and when phosphorescence is used, the internal quantum efficiency may reach 100%.
In general, as a phosphorescent host material, a carbazole-based compound and the like such as 4,4-dicarbazolybiphenyl (CBP) and the like are used, and as a phosphorescent dopant material, a metal-complex compound including heavy atoms such as Ir, Pt and the like is widely used.
However, in the case of CBP as a phosphorescent host material which is currently used, the compound has a low glass transition temperature (Tg) of about 110° C. and easily causes the crystallization in a device, thereby leading to a very short service life of about 150 hours in an organic EL device, which is problematic.